The invention relates to a method of operating a diesel internal combustion engine wherein fuel is injected into the combustion chamber of the engine to form a homogenous air/fuel mixture for the combustion of the fuel in the combustion chamber.
The prior art includes diesel internal combustion engines in which each cylinder is provided with an injector for direct fuel injection into the combustion space formed in the cylinder. The fuel is burnt with separately supplied combustion air, the compression of the air/fuel mixture by the piston triggering spontaneous ignition. Increasingly higher requirements are being made for low pollutant emissions from modern diesel internal combustion engines. The legally prescribed limits for pollutant emissions can often only be met by complex devices for aftertreatment of the exhaust gas, such as particle filters or catalystic converters. In particular, although influencing the diesel-specific ignition delay leads to a reduction in the emissions of nitrous gases, the generation of black smoke remains uncontrollable. Although particulate emissions can be reduced by optimizing fuel injection, for example with an increased injection pressure, this increases the combustion temperature and thus promotes the formation of nitrogen oxides.
EP 0 967 380 A2 discloses a method of operating a diesel internal combustion engine in which a reduction in pollutant emissions and, at the same time, in the generation of black smoke is to be achieved by the formation of a homogeneous mixture of fuel and combustion air. In this so-called pre-mixed mixture formation, the mixture is configured in such a way that a homogeneous mixture is formed in the combustion space no later than the time of spontaneous ignition, i.e. a largely uniform mixture ratio between the fuel and the combustion air exist in the combustion space. The fuel is injected into the combustion space at an early point in time during the compression stroke of the piston, and the mixture ignites at the end of the compression stroke.
By means of this combustion method, as in spark initiated-ignition internal combustion engines, the development of black smoke can be prevented and, at the same time, the formation of nitrous gases can be suppressed, thereby providing for reduced fuel consumption as well. In order to achieve the desired homogeneous mixture formation, the known method provides for a pre-injection of fuel in a period between the beginning of the the intake stroke and the middle part of the compression stroke of the piston and a subsequent main fuel injection. A control unit monitors the injection process involved in the two-part injection as a function of the operating state of the internal combustion engine. Here, the formation of black smoke is to be prevented by the fact that the main injection takes place repeatedly in a time interval at the end of the compression stroke and the beginning of the expansion stroke. Combustion is to be stabilized by taking account of the temperature of the fresh intake air.
It is the object of the invention to provide a method of operating a diesel internal combustion engine by which very low pollutant emissions can be achieved in combination with a uniform homogenized mixture formation.
In a method of operating a diesel internal combustion engine, wherein a homogeneous mixture is formed in the combustion space before a spontaneous ignition by means of fuel injected directly into separately supplied combustion air, the combustion air is admitted to the combustion space in a swirling flow about the longitudinal axis of the cylinder that can be adjusted as regards its swirl intensity by a control element. A control unit adjusts the swirl intensity in a manner coordinated with the injection parameters to provide a homogeneous mixture for highly effective engine operation with low emissions.
With this method, the mixture formation is completely finished with virtually uniform air ratios throughout the entire combustion space before the start of ignition. This prevents diffusive combustion. The homogenization of the mixture is varied by the control unit as a function of the engine operating conditions. The control element, which can be controlled for this purpose by the control unit, is preferably a control flap, which is arranged in an inlet duct of each cylinder. The swirl intensity of the combustion air flow, which comprises the component flows of both inlet ducts, can be adjusted to the desired value by restricting the inlet air flow in one inlet duct by means of the control flap arranged in the respective inlet duct.
Homogenization of the fuel/air mixture is achieved in combination with swirl control by the determination of the injection point and the quantity of fuel to be injected by the control unit. The quantity of fuel to be injected is dispensed, preferably, essentially in a continuous injection process of the injector. With the method according to the invention, a staggered pre- or pilot injection before the main injection is of no benefit since the maximum ignition delay of the diesel internal combustion engine can be achieved by combining control of the injection parameters with a control of the swirl intensity. It is regarded as particularly expedient if the injection process begins in a time window between about 30-40xc2x0 of crank angle before the top dead center of the crank shaft and ends about 10-20xc2x0 of crank angle after top dead center.
The introduction of the fuel provided for combustion takes place by a main injection and the fuel is preferably injected by the injector in a continuous injection process. To ensure a further reduction in pollutant emissions, especially post-oxidation of HC, CO and particulates, a staggered after injection can be provided, during which the proportion of the total quantity injected is smaller than the proportion in the main injection.
In an advantageous embodiment of the invention, a specific air ratio between the oxygen content of the combustion air and the quantity of fuel is set by recirculating exhaust gas at an adjustable recirculation rate. It is advantageous here if an air ratio specified for the respective operating point of the internal combustion engine is set. Such application values can be determined in advance, as can the settings of the swirl intensity and the injection parameters, and stored in a performance graph of the control unit for access as needed. At least in certain operating ranges of the internal combustion engine, the air ratio is set to values close to the stoichiometric ratio. This results in particularly low nitrogen oxide emissions.
The optimum recirculation rate, i.e. the proportion in the combustion air of exhaust gases recirculated and added to the fresh intake air, is determined by the control unit through appropriate adjustment of an exhaust-gas recirculation valve, which is arranged in an exhaust-gas recirculation line between the exhaust line and the inlet line of the internal combustion engine. By coordinated adjustment of the injection point, the swirl intensity of the combustion air flow and of the air ratio by means of the exhaust-gas recirculation rate to a specified value, the control unit can adjust the balance of conversion during combustion of the fuel to the optimum values. The control unit also specifies the course of combustion and the pressure gradient in the combustion space to provide values as a function of the engine operating point by means of associated settings of the air ratio, the swirl intensity of the combustion air flow and the injection parameters.
The method according to the invention provides a largely fully homogenous mixture in the ignition delay phase. It is possible with the method to establish a leaner-than-stoichiometric air ratio between the combustion air (fresh air/exhaust gas mixture) and the injected fuel. This mixture ignites in local zones with an almost stoichiometric air/fuel ratio. The ignition delay phase can be set to the optimum length for the operating point by means of the parameters that can be adjusted according to the invention, namely the injection rate, the injection point, the fresh air/exhaust gas ratio and the swirl intensity. In the process, the desired homogenization of the mixture and the position and size of the zones with ignition nuclei of an approximately stoichiometric mixture ratio are controlled. The premixing of the fuel with the combustion air, i.e. the completion of mixture formation before ignition, results in complete and low-soot combustion.
In a manner similar to internal combustion engines with spark ignition, it is possible in diesel internal combustion engines to achieve an air ratio close to the stoichiometric value to thereby eliminating the conflict between the aims of reducing particulate emissions and emissions of nitrogen oxides.